
This week we talk about polio, malaria, and smallpox.
We also discuss vaccines, the CDC, and the HHS.
Recommended Book: Remnant Population by Elizabeth Moon
Transcript
In 2000, the United States declared measles eliminated. That didn’t mean nobody in the country would ever catch it again, it meant the virus was no longer freely spreading within the US. New cases generally arrived with someone who had been infected elsewhere, and public health workers were able to stop those cases from turning into long-running chains of transmission via the systems, tools, and infrastructure they created.
In early October of 2026, Pennsylvania reported more than a thousand new measles cases this year, most of them part of an outbreak that began in April. As of October 7, the state had confirmed 1,078 cases across 41 counties, 207 hospitalizations, and five measles-associated deaths. Nationally, the Centers for Disease Control and Prevention, the CDC, had counted nearly 3,900 cases through the beginning of the month, making this the worst year for measles in the US in decades.
Pennsylvania’s latest update included 74 newly confirmed cases, though the state was careful to say that more than 40 of those people had become sick over two weeks earlier. So these were newly *reported* cases, not necessarily newly *infected* people—a distinction that matters when we’re trying to figure out whether an outbreak is speeding up or slowing down.
There’s a similar distinction being made with the death figures. Pennsylvania calls its five deaths *measles-associated*, using a definition that considers timing, a confirmed infection, medical evidence, and whether an unrelated cause might explain the death. The CDC’s national death count has not always matched the state’s, in part because the agencies publish on different schedules and review and tally deaths differently; so reporting on these sorts of things can sometimes be unintentionally fuzzy due to varying definitions and numbers.
What I’d like to talk about today is what it actually means to eliminate a disease, how measles has managed to claw its way back in a country that achieved that ‘eliminated’ milestone more than a quarter-century ago, and how vaccination, misinformation, and changing federal policy might shape what happens next.
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There are a few words public health workers and officials use that sound similar, but which describe substantially different achievements.
If a disease has been *eradicated*, it no longer circulates naturally anywhere in the world, as is the case with smallpox. The last known natural case was recorded in Somalia in 1977, and the World Health Organization declared the disease eradicated in 1980. There have been no natural smallpox outbreaks since. That’s a truly remarkable global achievement, and it’s why routine smallpox vaccination isn’t necessary for most people today.
*Elimination* is more local, and more conditional. It means sustained transmission of a disease has been interrupted in a defined place, like within a country. The pathogen can still exist elsewhere, and an infected traveler can still bring it in. Whether that introduction fizzles out or takes hold depends on what it finds when it arrives—how prepared that country’s government is, and how well vaccinated its population, how well funded its healthcare system, and so on.
Measles isn’t the only disease that’s been eliminated in the United States. The last US case of polio caused by *wild* poliovirus was in 1979, and the Americas were declared free of it in 1994. But in 2022, an unvaccinated person in New York developed paralytic polio caused by a vaccine-derived poliovirus, and genetically related virus residue was found in local wastewater. That was not wild polio somehow reappearing from the American past, it was a different type of poliovirus that latched onto a vulnerable person in a country where sustained polio transmission had long since been halted.
That virus was linked to the live oral polio vaccine used in some other countries, which can sometimes give rise to circulating variants where immunity is low. The inactivated polio vaccine used in the US cannot do that.
The US also eliminated sustained malaria transmission in the early 1950s, but mosquitoes capable of carrying the parasite still live in the US, and people returning from places where malaria is common can carry it across the border. In 2023, the CDC recorded ten locally acquired, mosquito-transmitted cases across four states—the first such cases in about twenty years. Ten cases doesn’t mean malaria has become endemic in the US again, it’s just further evidence that elimination of a disease in a region doesn’t mean reintroduction is impossible.
Smallpox, then, is an example of a disease that has been removed from natural circulation worldwide. Polio, malaria, and measles, in contrast, can be locally defeated, but regions in which that’s the case remain vulnerable to the movement of people, viruses or parasites, and that victory is reliant on the continued health of the health systems meant to detect and interrupt them.
Measles is an especially demanding test of those systems because it spreads so efficiently.
The virus moves through the air when an infected person breathes, coughs, or sneezes, and it can remain in the air, in a room in which someone has coughed, for up to two hours after that person has left. People can pass it on before the distinctive rash appears, and, according to the CDC, as many as nine out of ten nearby people who aren’t immune can become infected after exposure.
For some, the illness brings a fever, a cough, red eyes, and a rash, followed by recovery. For others, it causes pneumonia, swelling of the brain, or death. Young children, pregnant people, and those with weakened immune systems face higher risks, and this is why a measles outbreak isn’t just a rash going around, and why even just a few cases can generate an enormous amount of public health work: finding contacts, checking immunity, arranging vaccinations, and warning people who may have spent time in the same airspace. Measles can be painful and uncomfortable, but it can also be deadly, so all that effort is arguably well-invested.
The measles, mumps, and rubella vaccine—the MMR vaccine—is the main tool for preventing the spread of measles. One dose is about 93% effective against measles, and two are about 97% effective. That doesn’t mean vaccinated people can never get sick; these are estimates of how much the vaccine lowers the chance of infection. When most people in a community are protected, the virus also has fewer opportunities to reach those who cannot receive the vaccine or who haven’t yet reached the right age for their first routine dose.
Public health agencies often use about 95% coverage as a target for measles. That isn’t a magical national number, though, where a country is safe at 95.1% and doomed at 94.9%. It describes the level of protection usually needed within communities to make sustained spread difficult. A state can have a healthy-looking average while particular schools, neighborhoods, or social networks have far lower coverage and are thus at higher localized risk.
And that’s part of what makes the current trend concerning. CDC data show MMR coverage among American kindergarteners falling from 95.2% in the 2019-to-2020 school year to 92.4% in 2025-to-2026. About 280,000 kindergarteners lacked documentation of a completed MMR series in that latter year. Not all of them were necessarily unvaccinated (these sorts of records can be incomplete) but the direction of travel, and the concentration of missed vaccinations in some places and among some communities, leave more openings for the virus, which means everyone is at greater risk of infection.
There isn’t one explanation for all those missed doses. Some families have trouble getting appointments, taking time off work, or reaching a clinic. Others seek exemptions or have questions they don’t feel were answered well by medical …
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